This invention relates to an arrangement for degaussing color picture tubes having magnetic shields by use of a unitary degaussing coil in a tilted or skewed position.
Color television displays are often accomplished by means of shadow-mask picture tubes. In such tubes, a plurality of electron beams are directed from slightly different points of origin towards a screen coated with fluorescent phosphors. The phosphors are grouped in triads or groups of three, each member of which fluoreseces in a different primary color when illuminated by an electron beam. Each member of each triad is made responsive to a single one of the plurality of electron beams by means of a shadow-mask. The shadow-mask is a thin conductive shield with a multitude of perforations precisely located with respect to each triad of color phosphors. The shadow-mask ideally allows the phosphor of a particular color to be illuminated only by an electron beam originating at a particular location. Thus, with three electron beams originating in slightly different locations, each of the color phosphors of a trio is pure, i.e., is illuminated by only one electron beam, and gradations of color may be achieved by appropriate control of the source of electrons.
In the past, color television displays were sensitive to the influence of the earth's magnetic field. The magnetic field passing through the kinescope deflects the electron beams away from their intended paths and changes the apparent source of the electron beams arriving at the shadow-mask in a manner dependent upon the orientation of the picture tube relative to the earth's magnetic field, thereby impairing color purity. Early attempts to correct this problem involved the use of field neutralizing coils, as described in U.S. Pat. No. 2,921,226 issued Jan. 12, 1960 to Vasilevskis. In the field neutralization arrangement, a direct current is passed through one or more coils arranged about the kinescope in such a manner as to balance to zero or neutralize the earth's field. This required adjustment by an expert, and required readjustment if the television was moved to a new location.
Another approach to correcting color purity problems resulting from the earth's magnetic field involves the use of magnetic shields. In such an arrangement, a hollow shield in the form of the frustum of a cone surrounds the region through which the electron beam travels before reaching the shadow-mask and screen. The high permeability of the shield diverts extraneous magnetic fields away from the electron beam. It was found, however, that the permeable magnetic shield was capable of becoming magnetized and, when so magnetized, itself perturbed the color purity. Degaussing arrangements were then devised to demagnetize the shield and the shadow-mask. The degaussing is accomplished by passing an alternating current of initially large magnitude through one or more coils arranged about the shadow-mask and magnetic shield as described in U.S. Pat. No. 2,962,621 issued Nov. 29, 1960 to Fernald. The magnitude of the alternating current is then gradually reduced to zero and the magnetization of the shield and shadow-mask is reduced. A coil disposed along the periphery of the shadow-mask as illustrated by Fernald tends to produce a relatively small magnetic field near the center of the shadow-mask and screen. Thus, effective degaussing with such a coil requires currents which may be undesirably large.
In order to increase the magnetic flux through the magnetic shield and through the screen without excessive coil currents, arrangements such as those described by Norley in U.S. Pat. No. 3,322,998 issued May 30, 1967 and by Matsushima et al., in U.S. Pat. No. 3,872,347 issued Mar. 18, 1975 were devised. In the Norley and Matsushima arrangements, a pair of coils are arranged symmetrically on the picture tube, with each coil having a portion lying along the periphery of the shadow-mask and a portion extending towards the neck of the tube. The two-coil arrangements produce a magnetic field the lines of which are transverse to or across the longitudinal axis of the tube. This "cross axial" degaussing field approach using a pair of coils provides effective degaussing. However, the two coils require additional labor for mounting as compared with a single coil, and their interconnection may give rise to incorrect wiring. Furthermore, the two-coil arrangement tends to require a greater length of conductor than the single-coil type, and may respond to the deflection yoke field with currents which perturb color purity.
It is desirable to have a degaussing arrangement using a unitary coil requiring a relatively short conductor length which provides degaussing performance substantially equivalent to that of the two-coil system, combined with the simplicity and short conductor length of the single-coil type.